1. Field of the Invention
The present invention relates to apparatus and method for developing an electrographic record medium and more particularly to improved electrographic development structures and techniques for providing thorough developer cross-mixing (i.e. mixing transverse to the movement direction of the record medium being developed).
2. Description of Prior Art
In electrographic reproduction systems, e.g. electrophotography, one common form of image development involves contacting the imaging member (e.g. an electrostatically charged and imagewise exposed photoconductor) with a development mixture comprising toner marking particles and magnetic carrier particles which are selected to triboelectrically charge and attract the toner. The magnetic carrier particles with their triboelectrically attracted toner are magnetically attracted to a rotating brush which moves the development mixture (i.e. developer) from a supply zone to a development zone. There, a portion of the toner is attracted to the imaging member and the remaining toner returns with the developer to the supply zone (where it is remixed and returned for development of subsequent images). The magnetic brush can comprise stationary magnets within a rotating transport cylinder, rotating magnets within a stationary cylinder or devices where both cylinder and magnets rotate.
It is well known that removal of toner from the developer mixture, via image development, shifts the toner to carrier ratio of the mixture. Therefore, it is desirable to monitor the toner concentration carefully, and add toner to the mixture to maintain toner concentration within a limited range of variation. As toner is added it must be quickly mixed evenly throughout the developer in the supply zone, and various mixing means, e.g. transverse feed auger arrangements, are known for this purpose.
In order to maintain consistently good development of solid-area or continuous-tone images (in contrast to line and alpha-numeric images), another difficult problem must be confronted. Specifically, when the imaging member presents a high density, solid or continuous tone image section that extends across only a transverse (with respect to the feed path) portion of the development zone, there is a higher tone depletion from the adjacent transverse portion of the magnetic brush than from other brush portions. This transverse imbalance in toner depletion tends to persist for a period of time that extends into the next image development stages and results in relatively lower development of subsequent image portions that are aligned transversely with the preceding high density image portion.
Various structures and techniques have been suggested for minimizing the image artifact caused by such transient toner concentration imbalances. One prior art technique is to provide transversely slanting magnetic fields within the developer housing to continuously urge developer in one transverse direction as it moves along the development path. The cross-mixing of this technique tends to be slow in response and can cause an unbalanced distribution of developer across the transverse dimension of the development station.